I have a RTH7500 on a Payne Heat Pump.
My wiring scheme is as follows:
Red - R - 24VAC Hot
Yellow - Y/Y2 - Coll/Heat Stage 1
Green - G - Fan
Blue - C - Common
White - W/W1 - Heat Stage 2

What will make the fan run:
Closing between R and G?
Or Closing between G and C?

R-G powers the indoor fan circuit.

R and G will let your fan run
Are you sure it's a heat pump
Where is the reversing valve.
Recheck the wiring
Its can be 2 stage EFA + AC (heta strips in same unit)